562 2005. The Journal of Arachnology 33:562–568 THE SPERMATOZOA OF THE ONE-PALPED SPIDER TIDARREN ARGO (ARANEAE, THERIDIIDAE) Peter Michalik 1 , Barbara Knoflach 2 , Konrad Thaler 2 , Gerd Alberti 1 : 1 Zoologisches Institut und Museum, Ernst-Moritz-Arndt-Universita ¨t, J.-S.-Bach-Straße11/12, D-17489 Greifswald, Germany; 2 Institut fu ¨r Zoologie und Limnologie, Leopold–Franzens-Universita ¨t, Technikerstraße 25, A-6020 Innsbruck, Austria ABSTRACT. The species of the genus Tidarren are known for their one-palped males and outstanding copulatory behavior. In our ultrastructural observations of T. argo Knoflach & van Harten 2001, we show that this species possesses highly specific spermatozoa which differ from those found in other spiders: The nucleus of the sperm cell is strongly elongated and characterized by a conspicuous implantation fossa. The basis of the axoneme is located close to the acrosomal complex. The axoneme starts in front of the implantation fossa which extends deeply into the postcentriolar elongation. The implantation fossa is filled with dense staining globules and granules as in other theridiid species. Apart from these peculiarities, in T. argo the proximal centriole is located extraordinarily far away from the distal one. The encapsulated cleistospermia are surrounded by a thin secretion sheath. Remarkably, mature spermatozoa are not densely packed, but embedded in a copious secretion. Keywords: Spider sperm ultrastructure, nucleus, implantation fossa, centriole, secretion → Figures 1–4.—Late-stage spermatids in Tidarren argo. 1. Overview of part of the testis. Spermatids grouped together in cysts which are surrounded by extensions of the somatic cells (arrow). Scale bar = 10 m. 2. Longitudinal section of two spermatids. The ribbon-shaped nucleus coils several times around the axoneme as evident on the right spermatid. Arrow points to nuclear pores. Scale bar = 1 m. 3. The nucleus is surrounded by a manchette of microtubules. Nuclear canal runs along outer edge of the nucleus. The axoneme possesses a 9  2 +3 microtubular pattern. Scale bar = 0.5 m. 4. Longitudinal section of spermatids. Note the aberrant organization: Axonemal basis (dC) located in front of implantation fossa near acrosomal vacuole; implantation fossa with granular dense material; nucleus strongly elongated, its anterior part triangular. Scale bar = 1 m. Abbreviations: AV = acrosomal vacuole, Ax = axoneme, dC = distal centriole, Fl = flagellum, IF = implantation fossa, M = mitochondria, Mm = manchette of microtubules, N = nucleus, NC = nuclear canal, Sp = spermatozoa, V = vesicles. The theridiid spider Tidarren argo which was first described by Knoflach & van Harten 2001 from Yemen exhibits several peculiari- ties in exomorphology and behavior. The males amputate one palp some hours after the penultimate molt. Such self-amputation is known only from Tidarren and Echinotheri- dion species, but from no other spider (Kno- flach & van Harten 2000, 2001; Knoflach 2002). The reason for palp removal may be to increase locomotor performance as shown for T. sisyphoides (Walckenaer 1842) (see Ramos et al. 2004). In T. argo, the male dies during copulation and even becomes emasculated by the female. Immediately after insertion the fe- male twists off the single male palp, which then continues with sperm transfer discon- nected from the male (for details see Knoflach & van Harten 2001). Based on these outstand- ing features, the present study focuses on the fine structure of the spermatozoa, which are briefly compared with our own unpublished observations on other theridiid spiders. Tidarren argo from Yemen, Khamis Bani Sa’d, 15°11'N 43°25'E, were kept alive in Innsbruck, in plastic boxes at room tempera- ture. From this breeding stock, male speci- mens were dissected and fixed in 3.5% glu- taraldehyde in 0.1 M phosphate buffer, followed by postfixation in buffered 2% os-